We have created and implemented a microwave-powered chemical reaction interface which is attached to a mass spectrometer as a detection device. This interface atomizes molecules which enter it and, through the addition of a reactant gas, it converts the elements initially present into stable volatile polyatomic species whose mass spectra can be used to quantify those elements using conventional mass spectrometry. This approach appears applicable to a variety of samples including drugs and macromolecules and sampling techniques such as gas chromatography, high pressure liquid chromatography or direct introduction. Many elements, such as carbon, nitrogen, sulfur, halogens, or hydrogen, or specific nuclides, such as 14C or 15N can be monitored in a selective and sensitive manner using mass spectrometry. The goals of this project are to establish the pharmacological capabilities of this instrumental concept, and to develop the necessary hardware to accomodate a variety of input devices. The potential of the reaction interface/mass spectrometer concept suggests a wider range of applicability as a selective detector to both pharmacological studies and to chemical analysis in general than any other device presently available.